Candida species include fungal species that exist as natural commensals, but can become pathogenic if the host immune system has been compromised. Although Candida albicans is the most common fungal pathogen, infections due to Candida tropicalis have increased worldwide, establishing it as an emerging pathogen. Despite the importance of C. tropicalis infections and the high mortality rate associated with this species, little is known about its lifecycle and how it impacts disease. Significantly, my preliminary experiments have identified a novel form of phenotypic switching in C. tropicalis and that this switch regulates a cryptic program of sexual reproduction in this species. Based on these findings, this proposal will address how these newly discovered aspects of C. tropicalis impact its pathogenesis. As phenotypic plasticity can provide fitness advantages by allowing an organism to rapidly adapt to changing environments, it is important to study the phenotypic states of C. tropicalis and how they contribute to this species' ability to colonize and infect the mammalian host. The experiments outlined in Aim 1 will therefore examine the regulation of the white and opaque phenotypic states of C. tropicalis, and their differences in virulence and tissue specificity. For the second aim of this proposal, I will address how the mating cycle is completed in C. tropicalis, and how mating affects the virulence of this species. The experiments outlined in Aim 2.1 will elucidate the mechanism of ploidy reduction in the sexual cycle of C. tropicalis, as genetic variation that can result from the generation of recombinant progeny has implications for the organism's ability to survive and adapt to stressful environments. Thus, determining a mechanism for ploidy reduction and recombination will be important for analyzing the ability of this species to generate genetic diversity within a mainly clonal, but sexually reproducing, population. Moreover, information contained at the mating-type-like locus (MTL) is expected to influence the virulence of Candida species, as C. albicans strains that are MTLa/? have been shown to be more virulent than their homozygous a/a or ?/? counterparts. To determine if the information at the sex locus also alters the virulence of C. tropicalis isolates, Aim 2.2 will examine the competitive fitness of MTLa/a, ?/?, and a/? isolates in colonizing the host. These experiments will enable a more thorough understanding of how information at the sex-determining locus contributes to the ability of C. tropicalis isolates to colonize and infect the mammalian host. Overall, my discovery of switch-regulated sexual reproduction in C. tropicalis has important implications for how the species can generate both phenotypic and genetic plasticity. The experiments outlined in this proposal will determine how the novel white-opaque switch in C. tropicalis is regulated, and how phenotypic switching and sexual reproduction influence the organism's interaction with the host. Understanding how the different aspects of C. tropicalis biology affect its virulence will enhance our knowledge of how this species is so well suited to grow in the mammalian host both as a commensal and a pathogen.